This invention relates to an improvement in the process for the production of semiconductor materials by the vapor decomposition of a precursor compound of the semiconductor material and deposition of the desired semiconductor material. More specifically, this invention relates to an improved method of preheating the decomposition/deposition reactor.
High-purity semiconductor materials are a critical raw material in the electronics industry. The production of semiconductor materials, particularly silicon, via the decomposition of a gaseous precursor compound of a desired semiconductor material and deposition of the semiconductor material on a suitable substrate is a well-known, widely used process. The combined decomposition/deposition process comprises: (1) an electrically heated rod covered by an enclosure to allow operation under vacuum or pressure conditions; (2) a means for feeding a precursor compound of the desired semiconductor material and other reactant gases, as necessary, into the closed zone formed by the base member and the enclosure; (3) electrically heating the rod to a temperature sufficient to effect decomposition of the precursor compound of the desired semiconductor material and simultaneous deposition of the semiconductor material on the rod; and (4) handling any by-product gases and unreacted vapors of the precursor compound of the semiconductor material and other reactant gases. Representative examples of the apparatus and method described above are disclosed in several U.S. Pat. Nos.: 3,011,877, Schweickert et al., issued Dec. 5, 1961; 3,099,534, Schweickert et al., issued July 30, 1963; 3,147,141, Ishizuka, issued Sept. 1, 1964; 4,150,168, Yatsurugi et al., issued Apr. 17, 1979; 4,179,530, Koppl et al., issued Dec. 18, 1979; and 4,311,545, Bugl et al., issued Jan. 19, 1982.
One of the improvements that has been incorporated into the above-described process is the use of the desired semiconductor material as the material from which the electrically heated rods were fabricated. However, the nature of the semiconductor material renders the rods very poor conductors of electricity at ambient temperatures. To facilitate passage of electricity through the rods of the desired semiconductor material, the rods must be preheated to temperatures as high as 600.degree. C. In earlier designs of the above-described process the enclosure was fabricated out of quartz. The transparency or translucency of quartz allowed the use of external electrical resistance heaters to preheat the reaction zone and the semiconductor rods via radiant heat.
A further improvement in the above-described process was the fabrication of the enclosure from metallic materials of construction. Suitable materials of construction were found to satisfy the very stringent purity needs in the reaction zone. The use of metallic enclosures allowed construction of larger and larger decomposition/deposition reactors. These factors combined to complicate the means for preheating the rods of the desired semiconductor material to facilitate passage of electric current. Heating with external resistance heaters via radiant heat is not possible because of the metallic construction of the enclosure. Internal heaters to effect the preheating of the semiconductor rods requires complicated mechanical design and difficulty in selecting suitable materials that would not cause contamination and that would withstand the high temperatures and corrosive environment during the decomposition/deposition reaction.
Ishizuka in U.S. Pat. No. 3,147,141, issued Sept. 1, 1964, discloses a process for the manufacture of high-purity silicon by the decomposition of silane. Ishizuka discloses a metal reactor in which a conductive wire bridge is used as the heated surface in which silane is decomposed and silicon is deposited. The use of a conductive wire precludes the need for provisions to preheat the reaction zone and rod substrate to facilitate passage of an electrical current.
Yatsurugi et al., in U.S. Pat. No. 4,150,168, issued Apr. 17, 1979, disclose a method for producing high-purity silicon from the decomposition of silane. Yatsurugi et al., disclose the use of rods of semiconductor silicon as a heat source and substrate for the decomposition of silane and the deposition of silicon. Yatsurugi et al., disclose the use of an "air blast" passing through the reactor enclosure to facilitate heating the silicon rods to allow electrical current to pass and to heat the rods to an eventual reaction temperature of more that 800.degree. C. External cooling of the base member with water is disclosed. No means of externally preheating the reactor enclosure and the rods of semiconductor silicon is disclosed.
Koppl et al., in U.S. Pat. No. 4,179,530, issued Dec. 18, 1979, disclose the deposition of pure semiconductor materials, especially silicon, by thermal decomposition of gaseous compounds of the semiconductor material on heated carrier bodies of the semiconductor material. Koppl et al., disclose the use of an enclosure constructed of silver or silver-plated steel. Provisions are made for cooling of the enclosure with water. Koppl et al., state that the metallic enclosure of their invention is not permeable to heat radiation and heating of the carrier rod (semiconductor substrate) is accomplished by introduction of a heating finger into the reaction space. Koppl et al., further state that the ignition temperature (the temperature at which the carrier rods become conductive enough to be electrically heated to the decomposition temperature) is about 600.degree. C. for silicon. The heating finger disclosed by Koppl et al., is an electric heating coil enclosed in a quartz cylinder. To preheat the reaction space, the heating finger is lowered into the reaction space in the proximity of the carrier rods. Once the carrier rods are at the ignition temperature and electrical current can be passed through the carrier rods, the heating finger is removed from the reactor, and the opening in the metallic enclosure is sealed with a metal cover.
In the instant invention it was unexpectedly found that silicon thin rods could pass electrical current sufficient to heat the rods to decomposition temperature when the rods were preheated to approximately 250.degree. C. This lower preheating temperature, lower than taught in the art, facilitates more conventional means of heat transfer external to the reaction space. The ability to preheat the reactor externally eliminates the design and construction of complex, specialized pieces of equipment. Further, the use of external heating simplifies operation by eliminating the need to handle the internal heater. Further, in the case of a removable internal heater or the use of a flow of heated gas ("air blast" disclosed by Yatsurugi et al., supra,) introducing impurities during these operations would be a very possible occurrence. The unexpected finding that the reaction space need only be preheated to approximately 250.degree. C. facilitates the use of conventional heat transfer fluids rather than direct electric heating.
The primary objective of the instant invention is to provide an improved means for preheating the reactors for the chemical vapor decomposition of a precursor compound of a semiconductor material and deposition of the semiconductor material on an electrically heated rod of the semiconductor material. Another objective of the instant invention is the simplification of the reactor configuration of large metal decomposition/deposition reactors. A further objective is to reduce the possibilities of introducing impurities into the reactor space by minimizing the opening and closing of the enclosure and eliminating the introduction of extraneous materials, such as a gas for preheating the substrate rods.